Draw, trace, generate a NACA, handle a folded sheet, build a family, repair a polar: the complete aerofoil workshop.

Computing the polars

In any serious project a moment comes when no catalogue section will do: you have a section measured off an existing part, a folded sheet, a series 6 aerofoil imposed by a cavitation constraint, or a thickness that has to evolve from root to tip. Heliciel lets you build them.

• Tutorial outline:

1. Drawing an aerofoil
2. Tracing an existing shape
3. The NACA editors
4. Thick trailing edges
5. Thin sections and sheet metal
6. Building a family of aerofoils
7. Repairing a polar

1: Drawing an aerofoil

The drawing window gives you control over the points of the section, with leading edge and trailing edge zooms — the two zones where an irregularity of a tenth of a millimetre ruins a boundary layer calculation — and a set of correction tools.

The drawing window

2: Tracing an existing shape

Overlay mode displays an image in transparency under the section: a photograph of a section, a scanned drawing, a template survey. You follow the contour point by point. It is the simplest way to bring an existing blade into Heliciel, before computing its performance by reverse engineering.

Overlay mode

3: The NACA editors

The NACA 4-digit editor generates the classical sections, with a denser set of points at the leading edge. The NACA series 6 editor, laminar, was added for projects sensitive to cavitation: its flatter pressure distribution delays the suction peak that triggers cavitation.

The NACA 4-digit editor

The NACA series 6 editor

4: Thick trailing edges

A thick trailing edge is not a defect: on a very thick section, the Xfoil manual itself states that convergence is better with a thick trailing edge than with a sharp one. When a .dat file is imported, automatic trailing edge correction is no longer applied: the section enters exactly as it was measured.

5: Thin sections and sheet metal

Sheet metal blades and very thin sections do not converge under a code designed for thick aerofoils: that is the limit to be aware of when describing a folded sheet fan blade or the vane of a small turbine.

6: Building a family of aerofoils

The aerofoil family editor builds a progressive transformation from one section to another. That is exactly what a real blade needs: a thick, robust section at the root, a thin, efficient section at the tip, and a continuous evolution between the two — instead of an abrupt jump from one catalogue section to the next.

A family: from the root section to the tip section

7: Repairing a polar

A computed polar may contain an obviously wrong point — an unconverged stall, an isolated jump. The polar modification tools let you:

• modify, edit, correct the selected polar;
• enter a polar by hand: this is the way in for wind tunnel or towing tank measurements, when you have something better than a calculation;
• extend the polar with Xfoil by ±1°, down to negative Cl or to decreasing Cl;
• mirror the polars of symmetrical sections, and complete over 360° — useful for machines working outside their nominal range.

Correcting an outlying point

A polar modified by hand must be recorded in the study file: it is an input of the calculation, just like the fluid or the rotation speed.